The subject matter disclosed herein relates generally to switching devices and, more specifically, to safety switching devices for controlling guard locking devices. The present invention relates to safety switching devices capable of controlling a plurality of different kinds of guard locking devices and to a method for determining, by a safety switching device, which kind of a guard locking device is connected thereto.
Safety switching devices are, for instance used in industrial production environments in order to reliably and safely disconnect machines, such as a hydraulic press or a milling machine, if this is necessary for the safety of personnel or machinery. Safety switching devices typically have one or more input terminals to which input devices such as emergency stop buttons, two-hand operator buttons, guard door switches or light curtains can be connected. An evaluation and control unit evaluates the input signal from these input devices in a fail-safe manner and produces a switching signal on an output side being a function of same. The evaluation and control unit is also designed to operate in a fail-safe manner, for instance by providing redundancy. The switching signal controls actuators, in particular contactors for disconnecting the machine from a power supply and guard locking devices for locking/unlocking a movable guard such as a guard door.
The term “fail-safe” signifies that disconnecting the machine or locking the guard door must be ensured under all circumstances, even if a functional fault occurs within the chain comprising the input device, the safety switching device and the actuator. Safety switching devices must therefore be designed such that they ensure that the machine being monitored is always in a safe state, even in the event of the failure of one component, in the case of broken cables or other malfunctions. Accordingly, the term “safety switching device” in this context refers in particular to those devices and appliances which comply at least with category 3 of European standard EN 954-1, or comparable requirements.
Depending on the specific production environment, safeguards may be mandatory for controlling access to potentially harmful areas. In order to allow servicing personal to access these areas under certain conditions, guards or part of the guards may be moveable. Movable guards may be monitored with guard switches, for instance, to disconnect machines behind the guard when the guard is opened. Moreover, movable guards may be locked in the closed position by means of guard locking devices. In a safety application, safety switching devices may be employed for controlling and coordinating the processes of monitoring sensors (emergency buttons, guard switches, etc.), disconnecting machines, and unlocking corresponding guards.
For most types of guard locking devices, the unlocking action is conditional on the receipt of a certain electrical signal. However, guard locking devices may differ with respect to the kind of electrical signal required for performing the locking/unlocking action. Conventional guard locking devices require an electrical voltage to energize a lock release solenoid. More advanced guard locking device have an integrated safety circuit for controlling their operation and for implementing a fail-safe connection between the safety switching device and the guard locking device. The integrated safety circuit may regularly check the input signal(s) in order to detect a fault condition and to ensure that the guard lock is in a safe status when a fault occurs. Specifically, guard locking devices may be provided with OSSD (output signal switching device) terminals in order to allow for cross-wire or short-circuit fault detection. During regular operation, both OSSD signal lines deliver a DC voltage, such as 24 V. In certain time intervals, test impulses are modulated onto the DC voltage in order to detect a cross-wire or short-circuit fault. If a fault is detected, both signal lines are turned off.
A fail-safe connection between the safety switching device and the guard locking device may also be achieved by transmitting a dynamic signal on a single wire connection (single wire safety). The dynamic signal may be a clocked digital signal comprising a train of predefined pulses. Any deviation from the predefined pulse train, i.e., an invalid signal, indicates a connection fault, causing the guard locking device to return to or to remain in the safe status. Further details on implementing a fail-safe connection between two safety units by means of a single channel can be found in reference EP 2 383 762 A1, which is herewith incorporated in its entirety.